Investigation of thin films, heterostructures and devices of ceramic superconductors by means of high-resolution electron microscopy

Jia, C.

Book/Report

FZJ-2018-04454

Investigation of thin films, heterostructures and devices of ceramic superconductors by means of high-resolution electron microscopy

Jia, C. (Corresponding author)FZJ*

1993
Forschungszentrum Jülich GmbH Zentralbibliothek Verlag Jülich

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek Verlag, Berichte des Forschungszentrums Jülich 2809, IV, 149 p. (1993)

Please use a persistent id in citations: http://hdl.handle.net/2128/19471

Report No.: Juel-2809

Abstract: In this thesis a systematic study of the microstructure of YBa$_{2}$Cn$_{3}$O$_{7}$ thin films is presented by means of high-resolution electron microscopy (HREM). Most of the efforts are focused on the characterization of heterostructures of supercoducting YBa$_{2}$Cu$_{3}$O$_{7}$ and non-superconducting PrBa$_{2}$Cu$_{3}$O$_{7}$ and on YBa$_{2}$Cu$_{3}$O$_{7}$ films deposited on step-edge substrates. These specially designed structures exhibit a great potential for the electronic application of high-T e superconductors and for the investigation of the basic electric properties of the YBa$_{2}$Cu$_{3}$O$_{7}$ superconductor. In the experimental part (chapter 4) a technique for the preparation of crosssection specimens for high-resolution investigation is presented. Based on the conventional procedure a special process is developed in which the difference in the mechanical properties of the thin films and the substrates is taken intoaccount. A high quality and high yield of specimens are thus obtained. A high density of structural defects is detected in c-axis oriented YBa$_{2}$Cu$_{3}$O$_{7}$thin films. The most common ones are stacking faults which occur in (001) planes and are of an extrinsic type. Bordering these stacking faults Frank-type partialdislocations accommodate the difference in the stacking sequence between the faulted part and perfect part of the film. Dissociation of perfect dislocations is also found in the YBa$_{2}$Cu$_{3}$O$_{7}$ films. Besides, antiphase boundaries exist in the (100) and (010) planes of YBa$_{2}$Cu$_{3}$O$_{7}$. Detailed analysis of the defect structure is given in chapter 5. In chapter 6, interfacial deffects off c-axis oriented YBa$_{2}$Cu$_{3}$O$_{7}$/PrBa$_{2}$Cu$_{3}$O$_{7}$/YBa$_{2}$Cu$_{3}$O$_{7}$ triple layers are investigated. Since the individual layer thicknessexceeds the critical thickness misfit dislocations are introduced. In addition, a structural modulation which occurs on the side of the PrBa$_{2}$Cu$_{3}$O$_{7}$ layer is found. In the case of the YBa$_{2}$Cu$_{3}$O$_{7}$/SrTiO$_{3}$ interface, stand-off misfit dislocations are found to lie within the YBa$_{2}$Cu$_{3}$O$_{7}$ layer about 2 nm away from the interface. In chapter 7, a new technique is presented for chemically sensitive imaging of (YBa$_{2}$Cu$_{3}$O$_{7}$)$_{m}$/(PrBa$_{2}$Cu$_{3}$O$_{7}$)$_{n}$ superlattices. Zone-axis and off-zone-axis illumination are used to observe the compositional modulation of these superlattices. In both cases a chemically sensitive image contrast is obtained, which allows a distinction to be made between the Y and Pr occupant. The off-zone-axis technique has major advantages in practice. It yields a strong chemical signal over a much larger sample thickness range (about 16 nm) than the zone axis technique (a thickness range of approximately 4 nm). A characterization of (YBa$_{2}$Cu$_{3}$O$_{7}$)$_{m}$/(PrBa$_{2}$Cu$_{3}$O$_{7}$)$_{n}$ (m=1, n=6 and m=4, n=4) superlattices is described in chapter 8 using the technique mentioned above. The results show that high quality superlattices are obtained with sharp but terraced interfaces by high-oxygen-pressure dc-sputtering. No appreciable interdiffusion between the two compounds occurs. A superposition of the two compounds in the ab-plane is detected across the interface steps, due to kinks in the growth steps. This imperfection may have a significant effect on the electrical properties of the superlattices with m=1. On the basis of the observed morphology of the interfaces, which were once the growth surfaces a layer growth mechanism of the superlattice films is concluded. In chapter 9, the microstructure of YBa$_{2}$Cu$_{3}$O$_{7}$ films epitaxially grown on step-edge (001) SrTiO$_{3}$ substrates is characterized. A relationship between the microstructure of the film across a step and the angle the step flank makes with the substrate plane is found. On the flank of a steep step, the film grows with its c-axis perpendicular to that of the film on substrate surface so that two grain boundaries are formed. On the flank of a low-angle step, the film grows without any change in c-axis orientation across the step and without grain boundaries. These grain boundaries are responsible for the weak-link function of the stepedge junctions. In chapter 10, a detailed characterization of the structure and defects of the boundaries in YBa$_{2}$Cu$_{3}$O$_{7}$ film step-edge junctions on (001) LaAlO$_{3}$ substrate is presented. In the upper grain boundary, a [100] tilt axis and, on the average, a {013) habit plane alternates with a [010] tilt axis and a (103) habit plane. This alternating structure is caused by twinning in the orthorhombic film structure. The lower grain boundaries are found to be rather irregular consisting of a chain of (013)(013) and (010)(001) type segments which exhibits a tendency to tilt the whole habit plane toward the ab-plane of the flank film. Dislocations, stacking faults and misfit strains are also observed in or close to the boundaries. Grain boundary modelling yields a good agreement of the calculated images with the experimental ones and permits the atomic structure of the boundary to be determined. Possible implications for electrical properties of step-edge Josephson junctions are discussed on the basis of the microstructure of the grain boundaries.

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